1. Field of the Invention
This invention relates generally to a system and method for monitoring the anode bleed process of a fuel cell system and, more particularly, to a system and method for monitoring proactive and reactive bleeds of a fuel cell system that includes determining when the proactive bleed schedule should change to optimize the operation of the fuel cell system based on the number of reactive bleeds that have occurred as a result of an unexpected increase of nitrogen build-up in the anode-side of the fuel cell stack usually as a result of an aging system.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange member fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The MEAs are permeable and thus allow nitrogen in the air from the cathode side of the stack to permeate therethrough and collect in the anode side of the stack, referred to in the industry as nitrogen cross-over. Even though the anode side pressure may be higher than the cathode side pressure, the cathode side partial pressures will cause air to permeate through the membrane. Nitrogen in the anode side of the fuel cell stack dilutes the hydrogen such that if the nitrogen concentration increases beyond a certain percentage, such as 50%, the fuel cell stack becomes unstable and may fail. It is known in the art to provide a bleed valve at the anode exhaust gas output of the fuel cell stack to remove nitrogen from the anode side of the stack.
Some fuel cell systems employ anode flow shifting where the fuel cell stack is split into sub-stacks and the anode reactant gas is flowed through the split sub-stacks in alternating directions. In these types of designs, a bleed manifold unit (BMU) is sometimes provided between the split sub-stacks that includes the valves for providing the anode exhaust gas bleed.
An algorithm may be employed to provide an online estimation of the nitrogen concentration in the anode exhaust gas during stack operation to determine when to trigger the anode exhaust gas bleed. The algorithm may track the nitrogen concentration over time in the anode side of the stack based on the permeation rate from the cathode side to the anode side, and the periodic bleeds of the anode exhaust gas. When the algorithm calculates an increase in the nitrogen concentration above a predetermined threshold, for example 10%, it may trigger the bleed. This bleed, sometimes referred to as a proactive bleed, is typically performed for a duration that allows multiple stack anode volumes to be bled, thus reducing the nitrogen concentration below the threshold.
Another type of known bleed is referred to as a reactive bleed. In a reactive bleed an algorithm calculates the cell voltage and triggers a bleed when a cell voltage stack spread threshold or bounce threshold is exceeded. Stack spread is the difference between the maximum and minimum cell voltage per split sub-stack. Stack bounce is the absolute difference between the average cell voltage of both sub-stacks. The primary cause of cell voltage loss is nitrogen buildup in the stack. Thus, the typical purpose of the reactive bleed is to bleed the nitrogen accumulated in the anode side of the stack to improve the minimum cell voltage and reduce the stack voltage spread of the split stack system. Other less common causes of reactive bleed are known, such as drying out of the membrane or excess water in the anode.
As is well understood in the art, fuel cell membranes operate with a controlled relative humidity (RH) so that the ionic resistance across the membrane is low enough to effectively conduct protons. The fuel cell system is able to determine whether the membrane is too dry or too wet using a sensor that determines the high frequency resistance of the fuel cell. High frequency resistance (HFR) is the ohmic resistance of the membrane, which changes with hydration of the membrane. The higher the HFR the drier the stack, and the lower the HFR the more likely that excessive water in the anode is the cause of the reactive bleed.
One known anode exhaust gas bleed control algorithm determines the duration of the bleed based on a fixed time that would eliminate the desired amount of nitrogen. However, as a fuel cell stack ages, cell performance decreases and nitrogen bleeds are required more often. Therefore, those systems that employ a fixed bleed duration typically select a bleed duration for the mid-life of the stack as a suitable average for the entire stack life. However, such an anode bleed strategy is obviously not efficient for the whole life of stack where the bleed duration typically would be too long and too frequent when the stack is new and too short and too infrequent when the stack is near the end of its life. When the bleed is too long, the system operates inefficiently because a significant amount of hydrogen is being exhausted out of the anode exhaust. When the bleed is too short, the fuel cells begin to collapse, which triggers an anode bleed that normally may not be necessary. Typically the bleed duration is determined for different current density ranges of the stack, but which are fixed values through the life of the stack. Because the bleed trigger table is static, the calibration is biased in favor of cell stability to account for cell degradation. Thus, hydrogen is sacrificed using a fixed table in an effort to maintain cell stability over the duration of the stack life.